A heat-sensitive recording paper is a recording paper which forms images by utilizing the physical and chemical changes of a material or materials induced by the action of heat energy. Various processes involving heat-sensitive recording papers have been investigated.
Various heat-sensitive recording papers utilizing physical change or deformation of a material by the action of heat such as the so-called wax-type heat-sensitive recording paper have been known for a long time. Such materials are utilized for electrocardiographs. The utilization of the physical changes is described in U.S. Pat. No. 3,131,080.
In addition, various heat-sensitive recording sheets utilizing the chemical changes of materials induced by the action of heat have been proposed. In particular, the so-called two-component coloring type heat-sensitive recording paper is well known as described in U.S. Pat. Nos. 2,663,654, 2,663,655 and 2,967,785.
Heat-sensitive recording papers involve first order coloring, i.e., they do not require development. Therefore, the recording apparatus can be simplified. Furthermore, the cost of the recording paper and the recording apparatus are low, and the system is a nonimpact system giving no noise. In view of these advantages, such heat-sensitive recording systems have established a position as a low-speed recording systems. However, such heat-sensitive recording has a slow response speed as compared with other recording systems such as electrostatic recording systems.
The responding speed of a recording element is slow because the recording energy is heat. That is, heat conduction between a thermal head and a heat-sensitive recording paper which is brought into contact with the thermal head is insufficient. Accordingly, sufficient recording density is not obtained. A thermal head is composed of an assembly of dot-like electric resistant heating elements which generate heat in response to recording signals in order to melt and color the heat-sensitive color forming layer in contact with the thermal head. In order to obtain a clear high density recording with such a heat-sensitive recording system, the dot reproducibility must be good. More specifically, the energy generated from the thermal head must be effectively transferred to the heat-sensitive color forming layer during high-speed recording to create a coloring reaction therein and form dots corresponding to the dot heating elements of the thermal head. However, at present, only a small percentage of the heat generated from the thermal head is transferred to the heat-sensitive color forming layer. Accordingly, the efficiency of heat conductivity is extremely low.
Various efforts have been made to overcome this problem by altering both the recording apparatus and the recording paper. For example, various methods have been proposed for improving the smoothness of the heat-sensitive color forming layer so that the heat-sensitive color forming layer gets maximum possible contact with the thermal head.
Japanese Patent Publication No. 20142/77 describes a paper wherein the surface of the heat-sensitive color forming layer is surface-treated to 200 to 1,000 seconds by a Bekk smoothness. Japanese Patent Application (OPI) No. 115255/79 (the term "OPI" is used herein refers to a "published unexamined Japanese patent application") describes a paper having a Bekk smoothness of 200 to 1,000 seconds, wherein the heat-sensitive color forming layer can respond only to a thermal pulse of about 5 to 6 milliseconds. Accordingly, for high-speed recording below 1 millisecond, it is necessary to perform the smoothening treatment above 1,100 seconds by Bekk smoothness. If the Bekk smoothness is made higher than 1,100 seconds by pressing the heat-sensitive recording paper, color fog is formed by pressure. However, after the heat-sensitve recording layer is coated on the base paper increasing the smoothness to the Bekk smoothness above 500 seconds, if the Bekk smoothness is increased above 1,100 seconds by adjusting the surface of the heat-sensitive recording paper, the resulting recording paper can give high recording density without increasing fog formation. Japanese Patent Application (OPI) No. 156086/80 (British Pat. No. 2,051,391) describes a paper wherein surface roughness Ra and the gloss of the surface of a heat-sensitive color forming layer are controlled below 1.2 .mu.m and 25%, respectively.
The smoothness of a heat-sensitive color forming layer is improved by only calender treatment such as with a super calender, machine calender or gloss calender. The calender treatment may be applied to only the base paper, the base paper and the heat-sensitive recording paper, or only to heat-sensitive recording paper.
Calender treatment may improve the smoothness, thereby improve recording density. However, such treatment is accompanied by various faults. One fault is the formation of fog, that is, coloring of the recording paper by the occurrence of a coloring reaction during the surface treatment step. To eliminate the fog, the addition of granular wax has been proposed in Japanese Patent Publication No. 14531/75. However, waxes generally have a large heat capacity and heat of melting. The properties reduce the heat response speed of the heat-sensitive recording sheet.
Calendering treatment of the paper creates increased contact with the recording element. Accordingly, the recording layer may adhere to the element at the coloring portion causing peeling and noise, and causing the accumulation of fused matter of the heat-sensitive recording layer on the surface of the element. This may cause a reduction of recording density and a reduction in dot reproducibility and running property of recording paper reduces (so-called sticking).
Calendering also decreases the writability of the paper with respect to a pencil or ball-point pen.
Notwithstanding these faults, calendering treatment is used because the improvement in smoothness results in a substantial improvement of the paper response speed.